The present disclosure relates to pigment-based inks. More specifically, it relates to composite colors printed on media using pigment-based inks.
Pigment-based inks are often preferred over dye-based inks in applications requiring longevity and permanence. Pigment-based inks generally incorporate a plurality of tiny solid pigment particles having the color specified by the ink in a liquid vehicle. The pigment particles are insoluble, thereby providing a colored ink with better water fastness than dye-based inks. The aforementioned liquid vehicle in pigment inks is often an aqueous solution.
As pigment particles in a purely aqueous liquid vehicle tend to flocculate or clump together, pigments for ink formulation are commonly available in dispersions. In this context, dispersions are aqueous solutions containing pigment particles treated with a dispersing agent that greatly reduces the tendency of pigment particles to attract each other.
Electrostatic dispersants are one form of dispersing agent. Electrostatic dispersant particles have either a net positive or net negative charge and adhere to the surface of the pigment particles through adsorption, causing a thin layer of dispersant particles to form on the majority of the surface of each pigment particle. This layer in turn polarizes the surface of the pigment particles with either a net positive or net negative zeta potential, according to the polarity of the charge of the dispersant particles. As like charges repel each other, the pigment particles repel each other and are dispersed substantially evenly throughout the liquid vehicle.
In aqueous solutions, electrostatic dispersants provide far greater stability to pigment dispersions than other available dispersing agents such as steric dispersants. Thus electrostatic dispersants are optimal in many applications requiring pigment-based inks. Electrosteric dispersants take advantage of both electrostatic stabilization and steric stabilization simultaneously.
One particular challenge posed with the use of pigment-based inks is encountered when printing two or more colored inks together to form a composite color. On many substrates, one example being plain paper, composite colors formed from pigment-based inks exhibit poor color saturation or richness. Attempts to improve plain paper composite color saturation of pigment inks include over or under printing with a fixer ink. However this approach requires the use of a separate fixer pen, and can increase printing costs. Other attempts at improving composite color saturation of pigment inks involve the use of specially treated print media. Such media are generally expensive, and it is often preferred that a project be printed on generic media such as plain paper.